Alkaline treatment for detoxification of acetic acid-rich pyrolytic bio-oil for microalgae fermentation: Effects of alkaline species and the detoxification mechanisms

Bio-oil derived from Pyrolysis of lignocellulosic biomass contains appreciable amounts acetic acid, which can be used as substrate for growing microalgae Chlamydomonas reinhardtii. However, the toxic compounds in the bio-oil inhibit the cell growth. This work is to develop alkaline treatment methods to reduce the toxicity and improve fermentability of acetic acid rich bio-oil. When growing in raw bio-oil without any detoxification treatment, the algae can only tolerate up to 0.1 wt% of bio-oil. Treatment with KOH, NaOH and Ca(OH)2 significantly reduced the toxicity and consequently improved the fermentability of bio-oil. The bio-oil tolerant level by microalgae depended on the alkali species used. Among the three alkali species, Ca(OH)2 proved the most effective detoxification reagent. Inhibitory compounds such as furans, phenols, ketones, aldehydes, ethers, esters, alcohols were removed by Ca(OH)2 treatment through precipitation. The detoxification mechanisms by the Ca(OH)2 -based treatment were also explored. The synergistic effect of alkaline pH, high temperature, and presence of Ca2+ played an important role for the precipitation of those compounds, and the consequent detoxification. Collectively, the results shows alkali, particularly Ca(OH)2-based, treatment is an effective for reducing the toxicity of the pyrolysis derived bio-oil as fermentative substrate for microalgae growth. The microalgae can tolerant Ca(OH) 2-treated bio-oil up to 5.5 wt%, which was 55 times higher than algal tolerance level of untreated bio-oil.